$ 170. THE ACEPHALA. 185 
Genera: Orbicula, Terebratula, Lingula. 
ORDER II. LAMELLIBRANCHIA. 
Animals which are symmetrical and bivalved, and whose more or less 
closed mantle encloses two pairs of lamelliform tentacles and branchiae. 
SUB-ORDER I. MONOMYA. 
‘Famity: Osrracea. 
Genera: Ostrea, Anomia. 
Famity: Prctinga. 
Genera: Pecten, Spondylus, ‘Lima. 
Famity: Matgacta. 
Genera: Malleus, Perna, Crenatula. 
‘SUB-ORDER II. DIMYA. 
Famity: AVICULACEA. 
Genera: Avicula, Meleagrina, ‘Pinna. 
Famity: ARcacza. 
Genera: Arca, Pectunculus, Trigonia, Nucula. 
Famity: Natapes. 
Genera: Anodonta, Unio. 
Famity: Mytimacna. 
Genera: Mytilus, Modiola, Lithodomus, Tichogonia. 
Famity: CHaMacra. 
Genera: Chama, Isocardia. 
Famity: Carpracza. 
Genera: ‘Cardium, Lucina, Hiatella, Cyclas, Piscidium, Tellina,.Psam=- 
mobia, Venus, Cytherea, Venerupis, Mactra, Lutraria, Ungulina. 
Famity: Pytorrpae. 
Genera: Mya, Solen, Solenomya, Panopaea. 
SUB-ORDER II. INCLUSA. 
Famity: TEREDINA. 
Genera: Pholas, Teredo. 
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